Mobile communications essentially require compressed coding of digital information of speech and images, for efficient use of transmission band. Especially, expectations for speech codec (encoding and decoding) techniques widely used for mobile phones are high, and further improvement of sound quality is demanded for conventional high-efficiency coding of high compression performance. Also, since speech communication is used by the public, standardization of the speech communication is essential, and research and development being actively undertaken by business enterprises worldwide for the high value of associated intellectual property rights derived from the standardization.
In recent years, standardization of a scalable codec having a multilayered structure has been studied by the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) and MPEG (Moving Picture Experts Group), and a more efficient and higher-quality speech codec has been sought.
A speech coding technology whose performance has been greatly improved by CELP (Code Excited Linear Prediction), which is a basic method modeling the vocal tract system of speech established 20 years ago and adopting vector quantization, has been widely used as a standard method of ITU-T standard G.729, G.722.2, ETSI (European Telecommunications Standards Institute) standard AMR (Adaptive Multi-Rate), AMR-WB (Wide Band), 3GPP2 (Third Generation Partnership Project 2) standard VMR-WB (Variable Multi-Rate-Wide Band) or the like (see Non-Patent Literature 1, for example).
In a fixed codebook search of the above Non-Patent Literature 1 (“3.8 Fixed codebook-Structure and search”), a search of a fixed codebook formed with an algebraic codebook is described. In a fixed codebook search, vector (d(n)) used for calculating a numerator term of equation (53) is found by synthesizing a target signal (x′(i), equation (50) using a perceptual weighting LPC synthesis filter (equation (52)), the target signal being acquired by subtracting an adaptive codebook vector (equation (44)) multiplied by a perceptual weighting LPC synthesis filter from an input speech through a perceptual weighting filter, and a pulse polarity corresponding to each element is preliminary selected according to the polarity (positive/negative) of the vector element. Next, a pulse position is searched using multiple loops. At this time, a polarity search is omitted.
Also, Patent Literature 1 discloses polarity pre-selection (positive/negative) and pre-processing for saving the amount of calculation disclosed in Non-Patent Literature 1. Using the technology disclosed in Patent Literature 1, the amount of calculation for an algebraic codebook search is significantly reduced. The technology disclosed in Patent Literature 1 is employed for ITU-T standard G.729 and is widely used.